觀無量壽佛經疏 1.58

1.58

答曰：「若論眾生垢障，實難欣趣，正由託佛願以作強緣，致使五乘齊入。」

I answer: If we consider the defilements and hindrances of sentient beings, it is indeed difficult to aspire to and enter; it is precisely through relying on the Buddha's vows as a powerful condition that the five vehicles are caused to enter together.